Hydrocephalus is derived from Greek word hydro-water & cephalus - brain, referring to on increase in water content of brain i.e. an increase in the ventricular size. It results when there is an imbalance between the production of CSF & its drainage by the arachnoid villi. Three mechanisms account for the development of hydrocephalus.
Pathogenesis of hydrocephalus
Classification of HydrocephalusIt can be classified depending on level of obstruction.
Depending on physiological seepage via damaged subependymal lining resulting in periventricular ooze, it can be classified as:
CT findingsFigure: Normal CT scan
Figure: Meningitis with Hydrocephalus
Figure: Choroid plexus carcinoma with hydrocephalus
Causes of HydrocephalusThe
various causes of hydrocephalus are as follows:
Ventricular dilatation versus hydrocephalusCerebral atrophy produces dilatation of ventricular system with proportionated dilatation of cortical sulci & CSF spaces. (sylvian fissure) Any obvious disproportionality should point towards abnormal enlargement. In hydrocephalus, frontal horn of lateral ventricles become rounded, balloon shaped, whereas in atrophy, ventricular shape does not change. The anterior angle between two lateral ventricles may increase in hydrocephalus & subependymal ooze also may be present. All these features, with clinical correlation will differentiate between atrophy & hydrocephalus.
UltrasoundUltrasound being a safe, quick, non-invasive & repeatable modality, has a definite role in diagnosis of hydrocephalus. However, the ultrasound waves cannot penetrate the bony skull. It is still used in neonatal brain imaging where the open anterior fontanelle is the acoustic window. Hence, its use is limited between age group 6 months-2 years.
When hydrocephalus is diagnosed in intrauterine life, associated CNS / extracranial anomalies should be looked for such as - meningomyelocele, other neural tube defects like spina bifida or chiari malformation. Often hydrocephalus can be diagnosed in utero by 15 weeks gestation. In utero, an upper limit of 10mm for the ventricular atrium is considered significant and hydrocephalus can be suspected.
Neonatal hydrocephalus is easy to recognize by routine coronal & sagittal imaging. Thus, diagnosis & progression can be evaluated. Care must be taken so that changes in ultrasound sector depth do not result in apparent enlargment or decompression of ventricles related to magnification difference when different depth scales are used. Failure to do so may result in a false impression of changing hydrocephalus.
Ventricular/Hemispheric Ratio: V/H ratio is a standard method for grading Hydrocephalus. It is ideally taken at level of foramen of Monroe/ third ventricle in coronal section. Distance of the lateral wall of lateral ventricle from midline to the hemispheric width, if more than 0.35, is suggestion of ventricular enlargement.
Figure: Normal USG
Figure: USG Hydrocephalus
Severity of HydrocephalusThough degree of ventricular dilatation is estimated on imaging, clinical implications are unpredictable. Even with mild hydrocephalus there may be more neurological damage than expected.
On CT scan, prominent temporal horns are amongst the first indicators. Transverse diameter of third ventricle > 5mm is considered as abnormal. Ballooning of frontal horn with periventricular hypodensity is seen in obstructive hydrocephalus. In addition, ventricular SRC index may be used.
Ventricular SRC Index= Distance between Anterior tips of frontal horn/Bifrontal diameter at same level (from inner table of skull) (Nromal = 30% )
Ultrasound uses the same principle. However, grading as mild, moderate, severe is subjective.
Normal pressure hydrocephalusThis is an idiopathic form of communicating hydrocephalus, which features clinically as a triad of dementia, incontinence & gait disturbances. Symptoms may be relieved by ventricular shunting. Imaging appearances are nonspecific. The patient improves after repeated lumbar punctures. Persistence of intrathecal contrast for 48 hrs on CT cisternography is also suggestive of this condition.
Shunt complications and ImagingFollowing shunting, there is a drop in the size of the ventricles. However, even when tip of shunt tube is seen outside ventricles, ventricular size is not enlarged as this is tube with many side holes.
Complications of shunt surgery:
- Blocked shunt tube: Shunt tube is seen in situ the ventricles, but ventricles are still dilated.
Shunt-tube migration - Migration of shunt tube may be extracranial or intracranial (It occurs due to motion of patient's head).
- Subdural hematoma: These may occur when large ventricles are shunted - due to decrease in ventricular size, subarachnoid spaces increase, stretching the vein which tear, resulting subdural hematomas.
- Slit ventricle syndrome: Also called shunt dependency syndrome. It is seen due to inability of the ventricles to re-expand after shunting due to lack of compliance or adhesions.
- Isolated fourth ventricle: This occurs when there are exudates at aqueduct & fourth ventricular outlet or aqueductal stenosis. Following shunting, there is dilation of fourth ventricle due to the aqueductal stenosis. Hence, the fourth ventricle does not get decompressed.
Infection - Following shunting, there may be infection along the shunt tract leading to ventriculitis, which is seen as an enhancement along the ventricular wall.
Figure: Congenital Hydrocephalus with shunt
Definition of various terminologies used
Arrested hydrocephalus: This is a condition, in which the ventricular system is not actively dilating. A more appropriate term is compensated hydrocephalus.
Active hydrocephalus: Active hydrocephalus is marked by an increase of ventricular volume. The activity is recognized by pronounced clinical symptoms and by progression in follow up CT studies.
Hydrocephalus ex vacuo: With shrinkage of brain due to atrophy, ventricular system & sulcal spaces increase in size resulting in ex vacuo dilation.
Figure: Exvacuo dilation of ventricle
Colpocephaly: When occipital horn dilatation is more pronounced than the rest of the ventricular system, it is termed as colpocephaly. Probably, occipital white matter is most vulnerable to damage & hence ventricles get space to increase. It may also be related to stronger growth of the occipital calvaria.